Apparatus and method for non-invasively monitoring a subject&#39;s arterial blood pressure

ABSTRACT

Apparatus is disclosed for non-invasively monitoring a subject&#39;s blood pressure, in which a flexible diaphragm that encloses a fluid-filled chamber is compressed against tissue overlying an artery, with sufficient force to compress the artery. A first, relatively slow servo control system optimizes the amount of artery compression, which occurs at a mean transmural pressure of about zero, by modulating the volume of fluid within the chamber and noting the resulting effect on the pressure within the chamber. Since different pressure effects are realized according to the amount of artery compression, an appropriate control signal can be produced that provides the optimum mean diaphragm pressure. In addition, a second, relatively fast servo control system supplies the fluid to and from the chamber, so as to compensate for pressure variations within artery. This minimizes variations in the artery&#39;s effective diameter, whereby the pressure within the fluid-filled chamber closely follows the actual arterial pulse waveform.

This application is a division of application Ser. No. 08/766,810, filedDec. 13, 1996, which application is now: U.S. Pat. No. 5,848,970.

BACKGROUND OF THE INVENTION

This invention relates generally to apparatus and methods for monitoringa subject's arterial blood pressure and, more particularly, to suchapparatus and methods that monitor arterial blood pressurenon-invasively by applying a pressure sensor against tissue overlying anarterial blood vessel, to partially applanate or compress the vessel.

Two well known techniques have been used to non-invasively monitor asubject's arterial blood pressure waveform, namely, auscultation andoscillometry. Both techniques use a standard inflatable arm cuff thatoccludes the subject's brachial artery. The auscultatory techniquedetermines the subject's systolic and diastolic pressures by monitoringcertain Korotkoff sounds that occur as the cuff is slowly deflated. Theoscillometric technique, on the other hand, determines these pressures,as well as the subject's mean pressure, by measuring actual pressurechanges that occur in the cuff as the cuff is deflated. Both techniquesa determine pressure values only intermittently, because of the need toalternately inflate and deflate the cuff, and they cannot replicate thesubject's actual blood pressure waveform. Thus, true continuous,beat-to-beat blood pressure monitoring cannot be achieved using thesetechniques.

Occlusive cuff instruments of the kind described briefly above generallyhave been effective in sensing long-term trends in a subject's bloodpressure. However, such instruments generally have been ineffective insensing short-term blood pressure variations, which are of criticalimportance in many medical applications, including surgery.

One technique that has been used to provide information about short-termblood pressure variations is called arterial tonometry. One device forimplementing this technique includes a rigid array of miniature pressuretransducers that is applied against the tissue overlying a peripheralartery, e.g., the radial artery. The transducers each directly sense themechanical forces in the underlying subject tissue, and each is sized tocover only a fraction of the underlying artery. The array is urgedagainst the tissue, to applanate the underlying artery and thereby causebeat-to-beat pressure variations within the artery to be coupled throughthe tissue to the transducers.

The rigid arterial tonometer described briefly above is subject toseveral drawbacks. First, its discrete transducers are relativelyexpensive and, because they are exposed, they are easily damaged. Inaddition, the array of discrete transducers generally is notanatomically compatible with the continuous contours of the subject'stissue overlying the artery being sensed. This has led to inaccuraciesin the resulting transducer signals. In addition, in some cases, thisincompatibility can cause tissue injury and nerve damage and canrestrict blood flow to distal tissue. Another drawback is that suchrigid arterial tonometers have failed to correct for signal artifactsthat arise when the subject's arm is moved. This is a particular problemwhen the subject is exercising or otherwise ambulating.

Yet another drawback to the arterial tonometer described briefly aboveis its inability to continuously monitor and adjust the level ofarterial wall compression to an optimum level of zero transmuralpressure. Generally, optimization of arterial wall compression has beenachieved only by periodic recalibration. This has required aninterruption of the patient monitoring function, which sometimes canoccur during critical periods. This drawback is perhaps the most severefactor limiting acceptance of tonometers in the clinical environment.

Another device functioning similarly to the arterial tonometer includesa housing having a closed, liquid-filled chamber with one wall of thechamber defined by a flexible diaphragm. The device is applied against asubject's skin, with the flexible diaphragm pressed against the tissueoverlying a peripheral artery, e.g., the radial artery, and severalelectrodes located in separate compartments of the chamber sense volumechanges in the compartments that result from the beat-to-beat pressurevariations in the underlying artery. Although the device seeks toreplicate the arterial pressure waveform, it is considered to have arelatively low gain, making it unduly susceptible to noise. Further, thedevice must be calibrated periodically, during which time its continuousmonitoring of the subject's blood pressure waveform necessarily isinterrupted.

It should, therefore, be appreciated that there is a continuing need foran apparatus, and related method, for non-invasively and continuouslymonitoring a subject's blood pressure, with reduced susceptibility tonoise and without the need to intermittently interrupt the device'snormal operation for calibration. The present invention fulfills thisneed.

SUMMARY OF THE INVENTION

The present invention resides in an improved apparatus, and relatedmethod, for non-invasively monitoring a subject's arterial bloodpressure, with reduced susceptibility to noise and without the need tointermittently interrupt the pressure monitoring for calibration. Theapparatus includes a pressure sensor assembly that produces a pressuresignal indicative of the pressure applied against it and furtherincludes a coupling device that urges the pressure sensor assembly intocompressive engagement with tissue overlying the subject's blood vessel,to compress the vessel and ensure that pressure variations within thevessel are coupled through the tissue to the assembly. A controllercontrollably modulates the position of the pressure sensor assemblyrelative to the subject's blood vessel and monitors the resulting effecton the pressure signal, to produce a control signal that controllablypositions the pressure assembly relative to the subject's blood vesselsuch that the vessel is compressed by a prescribed mean amount. Thisoptimizes the coupling between the blood vessel and the sensor assembly.The pressure sensor assembly thereby senses the subject's blood pressurein an optimal manner.

More particularly, in one form of the invention, the pressure sensorassembly includes a base having an open cavity, with a flexiblediaphragm extending across the cavity to define a closed, fluid-filledchamber. The controller is a fluid volume controller configured tocontrollably supply a fluid to and from the closed chamber, to vary theposition of the flexible diaphragm relative to the tissue overlying thesubject's blood vessel. By monitoring the resulting effect on thepressure signal, the controller can supply an amount of fluid to thechamber such that the subject's blood vessel is compressed by theprescribed mean amount.

In a separate and independent feature of the invention, the fluid volumecontroller is configured to controllably supply fluid to and from theclosed chamber so as to substantially balance the variable force appliedto the diaphragm by any pressure variations in the patient's bloodvessel. This enables the vessel to assume an optimal geometrycontinuously, even during short term pressure variations due, forexample, to a heartbeat pulse. This enables the masking effect of thetissue overlying the vessel to be effectively eliminated, and it enablesa substantially increased gain and noise immunity to be provided.

An optimal vessel geometry is provided when the vessel's mean transmuralpressure is substantially zero. This optimal vessel geometry is achievedby configuring the fluid volume controller to modulate the fluid volumewithin the closed chamber while monitoring the resulting effect on thepressure signal, to produce a control signal that then is used toregulate the fluid volume such that the diaphragm is maintained at theoptimal position.

More particularly, the fluid volume controller includes a vesselgeometry sensor that produces a vessel geometry signal indicative of theactual geometry of the subject's blood vessel, an error signal generatorthat generates an error signal indicative of any difference between theactual geometry of the blood vessel and a prescribed geometry of thevessel, and a summer, responsive to the vessel geometry signal and theerror signal, that produces the control signal. A fluid supply suppliesthe fluid to and from the closed chamber based on this control signal.

The error signal generator can include a diaphragm position sensor, inwhich case the vessel geometry signal is indicative of the actualposition of the flexible diaphragm relative to the base. The diaphragmposition sensor can include a signal generator that generates amodulation signal coupled to the summer, for incorporation into thecontrol signal. This modulation signal causes the fluid volumecontroller to supply the fluid to and from the closed chamber in amanner that modulates the actual position of the flexible diaphragmabout a mean position. A correlator correlates the modulation signalwith the pressure signal, to sense any deviation of the diaphragm'sactual mean position from a prescribed mean position and to produce theerror signal.

In a more detailed feature of the invention, which can be implementedwhether or not the pressure sensor assembly is coupled by a fluid to thesubject's blood vessel, the modulation signal includes a succession ofalternating positive and negative lobes of substantially uniformamplitude, e.g., a sine wave, and it has a frequency substantiallygreater than the subject's expected heartbeat frequency. In addition,the correlator is configured to compare the amplitude and shape of thepressure signal during positive lobes of the modulation signal with theamplitude and shape of the pressure signal during negative lobes of themodulation signal, which is indicative of any deviation of the pressuresensor assembly's actual mean position from its prescribed meanposition. The correlator is configured to compare the amplitude of thepressure signal during a first stage of the subject's heartbeat (e.g.,systole) with the amplitude of the pressure signal during a second stageof the heartbeat (e.g., diastole), to sense any deviation of theassembly's actual mean position from its prescribed mean position.

In another separate and independent feature of the invention, acontroller is configured to controllably modulate the position of thepressure sensor assembly relative to the subject's blood vessel with aperiodic signal having a frequency substantially greater than thefrequency of the subject's expected heartbeat, and the controllermonitors the resulting pressure signal to produce a plurality ofpressure waveforms. Each such pressure waveform corresponds to adifferent phase of the periodic signal, which in turn corresponds to adifferent nominal amount of vessel compression. The controller isconfigured to select the particular one of the pressure waveforms thatderives from a transmural pressure of substantially zero, for example byselecting the particular waveform for which the pressure signal atsystole differs from the pressure signal at diastole by a maximumamount.

Other features and advantages of the present invention should becomeapparent from the following description of the preferred embodiments,taken in conjunction with the accompanying drawings, which illustrate,by way of example, the principles of the invention.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross-sectional view of a pressure sensor assemblyin accordance with the invention, in its prescribed position secured toa subject's wrist, with a flexible diaphragm and liquid-filled chamberdisposed adjacent to the subject's radial artery.

FIG. 2 is a block diagram of a first embodiment of a blood pressuremonitoring apparatus in accordance with the invention, incorporating thepressure sensor assembly of FIG. 1.

FIG. 3 is a graph depicting the typical sigmoid relationship between thetransmural pressure of a subject's radial artery and the artery'seffective diameter. Superimposed on the graph are several waveformsrepresenting the subject's actual blood pressure over severalheartbeats, along with waveforms representing the resulting changes inthe artery's effective diameter for conditions of under compression,optimal compression, and over compression.

FIGS. 4A-4C are graphs depicting the sigmoid curve of FIG. 3 and showingthe effects of a small volume modulation on the transmural pressure overthe time period of one heartbeat, for conditions of under compression,over compression, and optimal compression, respectively.

FIGS. 5A-5C are graphs of typical pressure signal waveforms over oneheartbeat that are provided by the blood pressure monitoring apparatusof FIG. 2, for conditions of under compression, over compression, andoptimal compression, respectively. The waveforms incorporate pressureoscillations that result from application of a 25-Hz modulation of thevolume in the liquid-filled chamber.

FIG. 6 is a block diagram of a second embodiment of a blood pressuremonitoring apparatus in accordance with the invention, incorporating thepressure sensor assembly of FIG. 1.

FIGS. 7A-7C are graphs of typical mean transmural pressure signalwaveforms over one heartbeat that are provided by the blood pressuremonitoring apparatus of FIG. 6, for conditions of under compression,over compression, and optimal compression, respectively. The waveformsincorporate pressure oscillations that result from application of a25-Hz modulation of the volume in the liquid-filled chamber.

FIG. 8 is a schematic diagram of a model of the subject's wrist, withthe pressure sensor assembly of FIG. 1 disposed adjacent to it. Thediagram schematic representations of the wrist's radial bone, radialartery, skin, and underlying tissue.

FIG. 9 is a block diagram of a third embodiment of a blood pressuremonitoring apparatus in accordance with the invention, whichincorporates the pressure sensor assembly of FIG. 1 and provides anaccurate depiction of a subject's arterial pressure waveform within aslittle as a single heartbeat.

FIG. 10A is a graph of an exemplary pressure signal produced by theblood pressure monitoring apparatus of FIG. 9, incorporating both asinusoidal modulation component and a subject heartbeat component.

FIG. 10B is a graph of ten reconstructed pressure waveformsreconstructed from the exemplary pressure signal of FIG. 10A by theblood pressure monitoring apparatus of FIG. 9.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

With reference now to the drawings, and particularly to FIGS. 1 and 2,there is shown a blood pressure monitoring apparatus having a sensorassembly 11 configured for attachment to a subject's wrist 13, with aflexible diaphragm 15 of the assembly compressively engaging the skin 17and other tissue 19 overlying the subject's radial artery 21. Bloodpressure variations within the artery are coupled through the tissue tothe diaphragm, and through a liquid-filled chamber 23 located behind thediaphragm, to a pressure sensor 25, to produce a pressure signal outputon line 27 that represents the artery's pressure waveform.

More particularly, the sensor assembly 11 includes a plastic base 29having the shape of an inverted, shallow cup, and a wrist strap 31 thatholds the base in its prescribed position on the subject's wrist 13. Theflexible diaphragm 15 extends across the opening of the cup-shaped base,to define the chamber 23 that carries a suitable working liquid, e.g.,water. When the sensor assembly is properly secured to the subject'swrist, it compressively engages the skin 17 and adjacent tissue 19 andcompresses the radial artery 21. The amount of compression affects thedegree of coupling of the pressure variations within the artery to theliquid-filled chamber 23 and, in turn, to the pressure sensor 25. Thepressure sensor is coupled to the liquid-filled chamber via alow-compliance conduit 33.

FIG. 3 is a graph depicting the sigmoidal relationship between theradial artery's transmural pressure and the artery's volume, oreffective diameter. Transmural pressure is the pressure across theartery wall, i.e., the pressure inside the artery minus the pressureoutside the artery. A high transmural pressure indicates that the artery21 is compressed by a small amount and thus has a relatively largeeffective diameter, whereas a low transmural pressure indicates that theartery is compressed by a large amount, i.e., has a flattened, ovalshape and thus has a relatively small effective diameter.

It will be noted in FIG. 3 that a normal arterial pressure variation ofabout 50 mmHg (i.e., 50 millimeters of mercury), due to a normalheartbeat, will cause-the artery's transmural pressure to vary by 50mmHg, as well. This assumes, of course, that the pressure outside theartery 21 remains unchanged.

When the artery 21 is compressed by only a small amount, i.e., when themean transmural pressure is highly positive, this 50 mmHg pressureexcursion will cause only a relatively small change in the artery'seffective diameter. This relationship is shown by the verticallyoriented waveform identified by the reference letter A in FIG. 3 and thehorizontally oriented waveform identified by the reference letter B.Similarly, when the artery is compressed by a large amount, i.e., whenthe mean transmural pressure is highly negative, this 50 mmHg pressureexcursion likewise will cause only a relatively small change in theartery's effective diameter. This relationship is shown by thevertically oriented waveform identified by the reference letter C andthe horizontally oriented waveform identified by the reference letter D.

However, if the artery 21 is compressed by an amount that corresponds toa mean transmural pressure near zero, this 50 mmHg pressure excursionwill cause a relatively large change in the artery's effective diameter.This relationship, which is shown by the waveforms identified by thereference letters E and F in FIG. 3, represents a maximum couplingbetween the arterial pressure and the sensor assembly 11. When theartery has this geometry, the arterial wall is unable to carry stressloads. It is desirable, therefore, to regulate the mean arterialcompression to match this optimum value.

To this end, the apparatus further includes a volume controller 35having a liquid source 37 that supplies liquid to and from the chamber23 of the sensor assembly 11, via a conduit 39. The volume controllerimplements a control scheme that regulates the volume of liquid withinthe chamber so as to provide optimal compression of the subject's radialartery 21. Under this control scheme, the volume controller activelyvaries the position of the sensor assembly's flexible diaphragm 15, soas to modulate, or dither, the effective diameter of the subject'sartery 21, and it analyzes the resulting effect on the pressure signalproduced by the pressure sensor 25. Different effects on the pressuresignal are provided according to the amount of artery compression.

The volume controller 35 effects this modulation of the diaphragm'sposition by applying a 25-Hz sinusoidal signal to the liquid source 37.This sinusoidal signal is produced by an oscillator 41 and coupled vialine 43 to a summer 45 and, in turn, via line 47 to the liquid source. A25-Hz frequency is selected, because it is generally higher than thehighest frequency components of interest in the artery's blood pressurewaveform.

It will be appreciated that, if the artery 21 is under compressed, thenthe ac pressure response to the 25-Hz volume oscillation will be largerduring systole than during diastole. This is because an incrementalarterial volume change (i.e., an incremental vertical movement along thesigmoid curve of FIG. 3) will induce a greater pressure change (i.e.,horizontal movement along the curve of FIG. 3) when the pressure ishighest, i.e., at systole, than when the pressure is lowest, i.e., atdiastole. This phenomenon is depicted in FIG. 4A, and also in FIG. 5A,which shows the pressure signal on line 27 aligned with the pressurewaveform representing one heartbeat, including both a systolic stage anda diastolic stage. It will be noted that the pressure signal's acamplitude is substantially greater during the systolic stage than duringthe diastolic stage.

Conversely, if the artery 21 is over compressed, then the ac amplitudeof the 25-Hz pressure oscillation will be larger during diastole thanduring systole. This is because an incremental change in the effectivediameter of the artery (i.e., an incremental vertical movement along thecurve of FIG. 3) will induce a greater pressure change (i.e., horizontalmovement along the curve of FIG. 3) when the pressure is lowest, i.e.,at diastole, than when the pressure is highest, i.e., at systole. Thisphenomenon is depicted in FIG. 4B, and also in FIG. SB, which shows thepressure signal on line 27 aligned with the pressure waveformrepresenting one heartbeat. It will be noted that the pressure signal'sac amplitude is substantially greater during the diastolic stage thanduring the systolic stage.

However, if the artery 21 is optimally compressed, then the acamplitudes of the 25-Hz pressure oscillation during the systolic andend-diastolic stages will be substantially the same. In addition, theoverall amplitude of the pressure oscillation is at a minimum when theartery is optimally compressed. This is because the sigmoid curve ofFIG. 3 is steepest at the point of optimal compression. These phenomenaare depicted in FIG. 4C, and also in FIG 5C, which shows the pressuresignal on line 27 aligned with the pressure waveform representing oneheartbeat.

The control system for implementing the control scheme described aboveis depicted in FIG. 2. In addition to the volume controller 35, thesensor assembly 11, and the pressure sensor 25 identified above, thecontrol system further includes a 25-Hz bandpass filter 49 that filtersthe pressure signal received on line 27 from the pressure sensor. Thefiltered signal, which incorporates only the 25-Hz component of thepressure signal, is supplied on line 51 to the volume controller. Ananalyzer 53 that is part of the volume controller receives the filteredsignal and compares its ac amplitude during systole with its acamplitude during diastole, to determine whether the artery 21 is undercompressed, over compressed, or optimally compressed. The analyzerproduces a corresponding error signal that is supplied on line 55 to thesummer 45, which sums the error signal with the 25-Hz modulation signal,to produce a control signal that controls the liquid source 37.

Operation of the control system automatically regulates the amount ofliquid in the liquid-filled chamber 23 of the sensor assembly 11 suchthat the assembly optimally compress the subject's radial artery 21. Forexample, if the analyzer 53 of the volume controller 35 determines thatthe ac amplitude of the filtered pressure signal is greater duringsystole than it is during diastole, then a positive error signal isproduced, which is coupled through the summer 45 to the liquid source37, to supply additional liquid to the sensor chamber and therebyincrease the compression of the artery. The opposite would occur if theanalyzer determines that the filtered pressure signal's ac amplitude isgreater during diastole than it is during systole.

To provide a visible display of the subject's arterial pressurewaveform, the apparatus further includes a 25-Hz band stop filter 57 anda display 59. The filter receives the pressure signal on line 27 fromthe pressure sensor 25, and it samples this signal at a rate of 50samples per second. This sampling is phased with the 25-Hz modulationsignal output on line 43 by the oscillator 41 such that it always occursat the modulation signal's zero crossings. The sampled pressure signalthen is coupled on line 61 to the display, for real-time display. Theuse of a low-compliance liquid within the sensor chamber 23 in contrastwith a relatively high-compliance gaseous fluid, provides improvedcoupling between the artery 21 and the pressure sensor 25.

In an alternative embodiment of the invention, not depicted in thedrawings, modulation of a pressure sensor assembly relative to thesubject's radial artery is achieved not by fluid volume modulation, butrather by a lever that is connected via an eccentric to a dc motor.Rotation of the motor at a frequency of, say, 25 Hz causes the sensorassembly to move toward and away from the patient's arterycorrespondingly.

In an independent feature of the invention, enhanced coupling of thepressure variance within the subject's radial artery 21 due toheartbeats is provided by actively maintaining the artery fixed at itsoptimal compression level throughout each heartbeat pulse. This isaccomplished by a second, faster control system, which servo controls,in real time, the amount of liquid in the chamber 23 of the sensorassembly 11 so as to counteract the effect of the arterial pressurevariations. Moreover, this servo control is effected in combination withthe first, slower control system, which regulates the sensor assembly toprovide the optimal mean artery compression, as described above. A blockdiagram of a blood pressure monitoring apparatus that implements bothsuch control systems is provided in FIG. 6. Major portions of thisapparatus are identical to the apparatus of FIG. 2 and are identified bythe same reference numerals.

With reference to FIG. 6, an optical position sensor 63 (e.g.,incorporating a light-emitting diode and a photodiode) is carried on thebase 29 of the sensor assembly 11, to provide a signal that indicatesthe position of the flexible diaphragm 15 relative to the base. Thediaphragm position can be assumed to be related directly to the positionof the subject's skin and, thus, to the effective diameter of thesubject's radial artery 21. The position signal-is coupled via line 65from the position sensor directly to the summer 45 of the volumecontroller 35, for incorporation into the control signal supplied online 47 to the liquid source 37.

Thus, for example, as the pressure within the artery 21 rises duringsystole, the position sensor 63 senses an increase in the artery'seffective diameter and the position signal therefore exhibits acorresponding increase. This increase is coupled to the liquid source37, which responds by supplying sufficient additional liquid through theconduit 39 to the chamber 23 of the sensor assembly 11 to counteract theincreased pressure. The result is that the artery's effective diameterremains substantially fixed throughout the arterial pulse. A loopbandwidth of about 100 Hz is preferred, which is significantly higherthan the 25-Hz modulation signal and the important harmonics of thearterial pulse.

As the liquid source 37 of the volume controller 35 regulates the volumeof liquid within the chamber 23 of the sensor assembly 11 so as tomaintain the effective diameter of the subject's radial artery 21substantially fixed, the pressure within the chamber necessarily willvary. This pressure variance within the chamber will closely match thepressure waveform of the artery.

Moreover, this close match between arterial pressure and chamberpressure will occur even if a gas were to be substituted for the liquidwithin the chamber 23. This is because the fast servo control systemprovides for a virtual low compliance, even though the fluid itselfmight have a relatively high compliance. In some applications, the useof a gas (e.g., air) within the chamber is preferred, to simplifycertain aspects of the control system's design. When a gas is used, agreater volume will need to be transported to and from the chamber;however, the servo control system can readily accomplish this.

FIGS. 7A-7C depict the pressure signal output on line 27 by the pressuresensor 25 for the time period of one complete arterial pulse, forconditions of under compression, over compression, and optimalcompression, respectively. It will be noted that the signals closelyfollow the actual arterial pressure waveform, which also is shown in thethree drawings. It will be noted that a pressure oscillation due to the25-Hz modulation signal is present in each of the depicted pressuresignals.

In the under compression condition (FIG. 7A), the 25-Hz component of thepressure signal is skewed with its positive lobes significantly smallerthan its negative lobes. Conversely, in the over compression condition(FIG. 7B), the 25-Hz component of the pressure signal is skewed with itsnegative lobes significantly smaller than its positive lobes. In theoptimal compression condition (FIG. 7C), the 25-Hz component haspositive and negative lobes of substantially the same amplitude. Inaddition, the total ac amplitude of the 25-Hz component of the pressuresignal is smaller than it is when either under compressed or overcompressed. This, of course, naturally follows from the shape of thesigmoid curve of FIG. 3; the incremental pressure will vary the least inresponse to an incremental artery diameter change at the point where thecurve is steepest, i.e., where the artery's transmural pressure is zero.

The blood pressure monitoring apparatus of FIG. 6 implements a differenttechnique from that of the apparatus of FIG. 2 to maintain the artery 21at its optimum amount of compression. In the FIG. 6 apparatus, theanalyzer 53 of the volume controller 35 examines the relative amplitudesof the positive and negative lobes of the 25-Hz component of thepressure signal, which is provided to the analyzer on line from the25-Hz bandpass filter 49. If the negative lobes are determined to belarger than the positive lobes, then it is deduced that the artery isunder compressed and an appropriate error signal is coupled to thesummer 45 and, in turn, the liquid source 37. Conversely, an errorsignal having the opposite sense is produced if the positive lobes aredetermined to be larger than the negative lobes, in which case it isdeduced that the artery is over compressed.

As mentioned above, the pressure signal produced by the pressure sensor25 closely follows the actual arterial pressure waveform. This benefitis due primarily to the servo control of the flexible diaphragm 15 ofthe sensor assembly 11 such that the effective diameter of the artery 21remains substantially fixed throughout each arterial pulse, as well asto the effective low compliance of the liquid within the chamber 23. Theschematic diagram of FIG. 8 will help to provide an understanding ofthis phenomenon.

The schematic diagram of FIG. 8 includes a simple, one-dimensionalspring model of the subject's wrist 13, including its radial artery 21,radial bone 67, and skin 17, as well as the tissue 19 located above andadjacent to the radial artery. Also included in the schematic diagram isa simple, one-dimensional spring model of the sensor assembly 11,including its wrist strap 31. The radial bone advantageously serves as afairly rigid backing for the radial artery, but the artery wall and thetissue above and adjacent to the radial artery are modeled as aplurality of resilient springs. The coupling between the wrist strap andthe base 29 of the sensor assembly also is modeled as a spring.

It will be appreciated that if the flexible diaphragm 15 is servocontrolled so as to minimize any variation in the effective diameter ofthe artery 21, then the attenuating effects of the various springs thatrepresent the wall of the artery, the skin 17, and adjacent wrist tissue19 is effectively eliminated. The only remaining element having anattenuating effect is the spring 69 that models the coupling between thebase 29 of the sensor assembly 11 and the wrist strap 31. Thisdramatically improves the coupling of the arterial waveform to thepressure sensor 25, whereby a substantially improved pressure signal canbe produced.

In another independent feature of the invention, an accurate depictionof the subject's blood pressure waveform, and an accurate measurement ofarterial pulse pressure amplitude, can be produced within as little as asingle heartbeat using a monitoring apparatus as depicted in FIG. 9,which implements a special control and monitoring algorithm. Someelements of the. apparatus of FIG. 9 correspond to elements of theapparatus of FIG. 2, and these elements are identified by the samereference numerals, but with an added prime (') symbol. Although theFIG. 9 apparatus is depicted as an open-loop system, it alternativelycould be implemented as a closed-loop system incorporating additionalelements corresponding to those of the apparatus of FIG. 2 and/or FIG.6.

More particularly, and with reference to FIG. 9, a liquid source 37' isconnected via a conduit 39' to a chamber defined in a pressure sensorassembly 11'. The sensor assembly includes a plastic base 29' and aflexible diaphragm that cooperate to form the chamber, and this assemblyis configured to be held in compressive engagement with a subject'swrist 13' by a wrist strap 31'. Actually, because the pressure waveformordinarily can be produced within merely a few seconds, the wrist strapcan be eliminated and the apparatus operated simply by pressing thediaphragm manually against the subject's wrist.

A clock circuit 71 supplies a 25-Hz sine wave signal on line 43' to theliquid source 37', to cause to liquid to be supplied to, and drawn from,the chamber of the pressure sensor assembly 11' in a sinusoidal fashion.If the assembly is being urged against the subject's wrist 13' such thatthe arterial wall is partially compressed, this volume modulationordinarily will cause a corresponding sinusoidal pressure variationabout a mean pressure within the chamber. This pressure variation isdetected by a pressure sensor 25', which is connected to the chamber viaa conduit 33', to produce an analog pressure signal output on line 27'.

One representative waveform for the analog pressure signal on line 27'is depicted in FIG. 10A, which corresponds to a situation in which thepressure sensor assembly 11' is being urged against the subject's wrist13' with a mean pressure of about 80 mmHg. It will be noted that thepressure signal includes not only a 25-Hz modulation component, but alsoa heartbeat component. The apparatus of FIG. 9 functions effectively todemodulate this waveform and thereby to produce an accuraterepresentation of the subject's actual blood pressure waveform.

The apparatus of FIG. 9 achieves this demodulation by sampling theanalog pressure signal on line 27' at a sample rate that is an integralmultiple of the 25-Hz modulation frequency. At a sample rate of 250 Hz,for example, ten samples would be provided for each modulation cycle.Since the first sample of each successive modulation cycle is producedwhile the 25-Hz modulation component is at the same level, it followsthat these first samples can be associated together to produce adepiction of the subject's variable arterial pressure for thatparticular level. The same is true for each of ten separate sets ofsamples. Thus, ten separate pressure waveforms can be generated, eachrepresenting a different nominal arterial compression.

Exemplary versions of these ten separate pressure waveforms are depictedin FIG. 10B. It will be noted that the waveform having the lowestmagnitude represents the pressure samples that are made when the 25-Hzmodulation waveform is at its lowest value, and that the waveform havingthe highest magnitude represents the pressure samples that are made whenthe 25-Hz modulation waveform is at its highest value. The particularone of the ten waveforms that provides the greatest difference betweenthe systolic and diastolic values is deemed to be produced when thesubject's artery is compressed by the particular amount that providesoptimal coupling to the pressure sensor assembly 11'. In the exemplarycase depicted in FIG. 10B, this particular waveform is depicted by thebold reference line G.

It will be appreciated that the particular one of the ten waveforms thatis considered optimal will vary depending on the nominal pressureapplied by the pressure sensor assembly 11' to the subject's wrist 13'.If the nominal pressure is relatively low, then the optimal waveformwill likely be one of the higher-level waveforms, which are producedwhen the 25-Hz modulation signal is at or near its positive peak. On theother hand, if the nominal pressure is relatively high, then the optimalwaveform will likely be one of the lower-level waveforms, which areproduced when the 25-Hz modulation signal is at or near its negativepeak.

With reference again to FIG. 9, the successive samples of the analogpressure signal on line 27' are produced by a sample-and-hold circuit73, under the control of a 250-Hz clock signal supplied on line 75 fromthe clock circuit 71. The resulting samples are supplied on line 77 toan analog-to-digital converter 79, which produces a succession ofdigital words that are coupled on lines 81 to a suitable processor 83.The processor associates together the successive samples, as discussedabove, to produce the ten separate waveforms, and it ascertains theparticular waveform that corresponds to the optimum level of nominalartery compression. This particular waveform is then coupled via line 85to a display 59'.

It will be appreciated that results similar to those provided by theapparatus of FIG. 9 alternatively could be provided by substituting atonometric sensor assembly for the depicted fluidically coupled sensorassembly. The important feature to be retained is that the position ofthe A sensor assembly relative to the subject's artery be modulated andthat the resulting pressure signal be sampled at a rate that is anintegral multiple of the modulation frequency.

It should be appreciated from the foregoing description that the presentinvention provides an improved apparatus for monitoring a subject'sblood pressure, non-invasively, in which a flexible diaphragm iscompressed against tissue overlying an artery with sufficient force tocompress the artery by an amount that optimally-couples pressurewaveforms within the artery. In addition, the amount of liquid containedwithin a chamber located behind the diaphragm is servo controlled, tocompensate for pressure variations due to arterial pulses. Thisminimizes variations in the artery's effective diameter, whereby thepressure within the liquid-filled chamber is made to closely follow theactual arterial pulse waveform.

Although the invention has been described in detail with reference onlyto the preferred embodiments, those skilled in the art will appreciatethat various modifications can be made without departing from theinvention. Accordingly, the invention is defined only by the followingclaims.

We claim:
 1. Apparatus for non-invasively monitoring the pressure withina subject's blood vessel, comprising:a pressure sensor assembly thatproduces a pressure signal indicative of the pressure applied againstit; a coupling device configured to urge the pressure sensor assemblyinto compressive engagement with tissue overlying a subject's bloodvessel, to compress the vessel and ensure that pressure variationswithin the vessel are coupled through the tissue to the pressure sensorassembly; and a controller that controllably modulates the position ofthe pressure sensor assembly relative to the subject's blood vessel witha periodic signal having a frequency substantially greater than thefrequency of the subject's expected heartbeat, wherein the controllermonitors the pressure signal to produce a plurality of pressurewaveforms, each corresponding to a different phase of the periodicsignal.
 2. Apparatus as defined in claim 1, wherein the controllerfurther is configured to select the particular one of the pressurewaveforms that derives from a transmural pressure of substantially zero.3. Apparatus as defined in claim 2, wherein the controller further isconfigured to select the particular one of the pressure waveforms thatderives from a transmural pressure of substantially zero by selectingthe particular waveform for which the pressure signal at systole differsfrom the pressure signal at diastole by a maximum amount.